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Χωροχρονική Κρύσταλλος
Κρύσταλλος Space-time Crystal thumb|300px| [[Χρονική Κρύσταλλος ]] thumb|300px| [[Κρυσταλλικό Σύστημα ]] - Ένα Χωροχρονικό Μόρφωμα Ετυμολογία Η ονομασία "Κρύσταλλος" σχετίζεται ετυμολογικά με την λέξη "[[]]". Εισαγωγή A space-time crystal, time crystal, or four-dimensional crystal, is a theoretical structure periodic in time and space. It extends the idea of a crystal to four dimensions. The idea was proposed by Frank Wilczek in 2012. His speculation was that a construct would have a group of particles that move and periodically return to their original state, perhaps moving in a circle, and form a time crystal. In order for this perpetual motion to work, the system must not radiate its rotational energy. This type of motion is distinct from that of persistent currents in a superconductor, wherein the rotating Cooper pairs are not time crystals because their wave functions are homogenous, meaning time translational symmetry isn't broken. Symmetry would be spontaneously broken in Wilczek's ring system if its ground state still involves continuous movement. Tongcang Li and others proposed a system with beryllium ions circulating in a magnetic ion trap at about 10−9 K. Wilczek also suggested that a computing device could be possible with different rotational states representing information, and maybe different kinds of ions. Since this construct is in the lowest energy state it could in principle survive the heat death of the universe and continue forever. In May 2013 researchers announced they will attempt to build a component of a space-time crystal, by making a rotating ring of calcium ions. Their location will be confined by electric field, and rotation in a ground state will be forced by a magnetic field. Unwanted disturbances will be minimized by reducing the temperature to 1 μK by way of laser cooling. The ion trap will be 100 μm wide. Possible rotation of the ion ring will be demonstrated by using a laser to electronically excite one of the trapped ions. Patrick Bruno has criticized this concept, arguing that Wilczek's rotating state is not the ground state of the system. He derives the supposed true, non-rotating ground state. In August 2013 Bruno presented arguments that indicated rotating ground-state systems are impossible. Haruki Watanabe and Masaki Oshikawa formalised the definition of space-time crystals from a ground state only requirement to also include states in thermal equilibrium. The definition used the correlation of the local order parameter at different points in space and time. This correlation in a time crystal shows a periodic oscillation as a function of time difference even as volume is increased to infinity. Next they claimed to show that time translation symmetry cannot be broken thus proving that time crystal do not exist. With the extension of the definition to crystals with a finite temperature, the Lieb-Robinson bound is used to show that for small enough time intervals the correlation over a time difference has an upper bound that tends to 0 as the volume increases. A similar idea called a choreographic crystal has been proposed . In March 2016 researchers Else, Bauer and Nayak proposed that a non-equilibrium driven system called a "Floquet-many-body-localized driven system" could have broken time symmetry. Experimental Space-time crystals In October 2016, researchers at the University of Maryland, College Park claim to have created the world's first discrete time crystal. Using the idea from the March proposal, they trapped a chain of 171Yb+ (ytterbium) ions in a Paul trap, confined by radio frequency electromagnetic fields. One of the two spin states was selected by a pair of laser beams. The lasers were pulsed, with the shape of the pulse controlled by an acousto-optic modulator using the Tukey window to avoid too much energy at the wrong optical frequency. The hyperfine electron states are called 2S1/2 |F=0, mF = 0⟩ and |F = 1, mF = 0⟩. The different energy levels of these are very close, separated by 12.642831 GHz. Ten Doppler cooled ions were used in a line 0.025 mm long. The ions were coupled together. The researchers observed a subharmonic oscillation of the drive. The experiment also showed "rigidity" of the time crystal, where the oscillation frequency remained unchanged even when the time crystal was perturbed. However if the perturbation drive was too great, the time crystal "melted" and lost its oscillation. Υποσημειώσεις Εσωτερική Αρθρογραφία * Χρονοκάτροπτο * Ισλανδική Κρύσταλλος * Κρυσταλλική Κατάσταση * Κρυσταλλικό Σύστημα Βιβλιογραφία *H. Brown, R. Bulow, J. Neubuser, H. Wondratschek and H. Zassenhaus, Crystallographic Groups of Four-Dimensional Space. Wiley, New York, 1978 * * *Frank Wilczek 11 July 2012 [http://arxiv.org/abs/1202.2539 Quantum Time Crystals] * Ιστογραφία *Ομώνυμο άρθρο στην Βικιπαίδεια *Ομώνυμο άρθρο στην Livepedia *outlive the universe * the-quest-to-crystallize-time * time crystals Κατηγορία:Υλικά Μορφώματα